This invention relates generally to video decoder systems and more particularly to comb filters used in video decoder systems.
As is known in the art, a composite video signal has luminance (Y) and color difference signals (U & V) combined together into one signal. The U and V signals also referred to as chrominance signals are quadrature modulated onto a subcarrier and added to the luminance signal to produce the composite video signal. By using a high frequency subcarrier, the color signal only disturbs the high frequency portion of the luminance signal.
A typical decoder architecture employed to decode the composite video signal generally includes an A/D converter which samples the composite signal and feeds the output of the A/D converter to a notch filter. The notch filter removes the color information centered about the color subcarrier frequency leaving the luminance signal Y. The composite video signal is also fed to a quadrature demodulator and a pair of low pass filters which recover the color difference signals U and V. The quadrature demodulator includes a phased lock loop which locks onto the color burst signal and controls a direct digital synthesizer which generates a local copy of the color subcarrier. The local copy of the color subcarrier is used to demodulate the composite signal fed to the demodulator. With this simple decoder architecture mentioned above, the high frequency luminance is still present as a cross-luminance term in the U and V signals. That is, the simple decoder arrangement mentioned above cannot distinguish between luminance and color information centered on the color carrier. One approach used to overcome this problem is to add to the simple decoder architecture a comb filter. The comb filter is used to remove or reduce the cross-luminance information in the U and V signals. Several types of comb filters can be used at the demodulation stage in the decoder. The comb filters fed by the U and V components from the output of the decoder can reduce the high frequency luminance.
It is also known in the art that currently there are two popular standards for video encoding. One standard called the NTSC standard and the other is called the PAL standard. The essential difference between the NTSC and the
standard is that the phase angle between the color difference components on successive lines is 180.degree. for the NTSC standard and -90.degree. for the PAL standard and that the NTSC standard has a subcarrier frequency of 3.58 MHz, whereas the PAL standard has a subcarrier frequency of 4.43 MHz.
For the PAL standard a number of comb filter implementations can be used after demodulation. Each of them, however, has certain disadvantages. For example, in a simple one horizontal line PAL comb filter, the color difference components (U, V) can be simply averaged over a current line and the previous line. The problem with this arrangement, however, is that this comb filter only cancels hue error but does not cancel cross-luminance error. A second PAL comb filter which averages a signal over a current line and a second previous line has the disadvantage that it only cancels the cross-luminance error but does not cancel the hue error. A three horizontal line comb filter which operates over four lines can cancel both the U and cross-luminance errors; however, it has a significant disadvantage that it greatly reduces the vertical bandwidth and needs to store three horizontal lines of video information. Thus, this implementation is more expensive than the foregoing implementations.